Neurotransmitter systems have been implicated in many higher-order functions, both cognitive and emotional, but specific sites and mechanisms of action have not been identified. Studies to localize specific receptors for certain neurotransmitters in the monkey have been undertaken to attack these problems. Findings following lesions of the amygdala suggest that there may be opiatergic projections from the amygdala to higher-order sensory processing areas, such as the anterior insula and orbitofrontal areas. Developmental studies show that whereas limbic cortical areas and most subcortical regions have adult-like opiate receptors at birth, neocortical areas have simplied and undifferentiated binding patterns unlike the patterns seen in adults. Metabolic studies link the level of mu opiate receptors to the rate of protein phosphorylation in the F1 band. Since phosphorylation of F1 protein has been correlated with learning, these results suggest that opiates may help control the learning-related phosphorylation process. Autoradiographic localization of benzodiazepine and beta-carboline receptors in monkey shows that both drugs bind with apparently identical distributions, implying that they both act on the same brain regions to produce their effects on anxiety. Autoradiographic localization of nicotinic and muscarinic cholinergic binding sites in monkey suggests that, in the cortex, nicotinic sites are situated so as to modulate incoming afferent information, whereas muscarinic sites are more likely to modulate intracortical processing.